Instructions
Insert the USB cable from the hard drive into an open USB port on your computer.
Click the Windows Start button and click "Control Panel." Click the "System Security" option and click the "Administrative Tools" option.
Click the "Disk Management" option under the "Storage" section. Right-click "USB Drive" and click "Format." Click the "Format Type" menu and click "FAT64."
Click "OK," then click "OK" in the prompt that appears to format the hard drive to file system type that is readable on your Xbox 360 Slim.